So we finally have a singer in the band who has a good voice but has trouble coming up with vocals for our complete instrumental song.
The song is is very good like it is imo so the song isn't the problem. The parts fit together nicely. It has a great feel.
But the singer just can't figure out how to sing it except sing almost exactly the same thing the guitars play. It's more of a riff song, not so many chords.
Do you have any tips? Is there a certain way to write vocals on a song like this or should you just sing whatever feels right and it's all about creativity and what sounds the best on the riffs?
(it's a hard rock song)

Sure he's the vocalist but there is nothing wrong with another member thinking of a melody and showing it to him. Even if you can't hit or sing the notes to show him, you play guitar, an instrument capable of melody, show him via that.

__________________When I was eleven I broke the patio window and my mother sued me... She's always been a very aggressive litigator.

If they are originally meant to be instrumental songs, then maybe you just need to write new songs or re arrange the whole songs. Our band has instrumental songs that wouldn't work with vocals. Or they would if the structure was different and we changed the parts a bit. But yeah, if they are instrumental songs, they are instrumental songs (unless they are songs that really aren't meant to be instrumentals, they just lack vocals). But coming up with a good melody isn't that easy. If you have ideas for the melody, show them to your singer. Do you already have lyrics for the song?

Not every band member needs to - or even should - write their own parts. Most of the time when I write songs, I write all parts. I of course let the others change the parts a bit if it fits the song and sounds good. You shouldn't demand your band members to come up with all their parts (I'm not sure if you are doing it, but I'm just saying).

I find that adding spacing on the vocals with riffs below it help. Start off with a few main notes and fill it in. That song clearly isnt natural to him and someone can def help and even sing out of key to give a melody. Listen to Van Halen a bit even simple like "panama" and you'll see how the riff is played and the singer just uses the notes in the chords and focuses on that, not the riff below it. Thats best explanation I have right now.